Optical fibers are typically coated with a dual layer coating system, the main purpose of which is to protect the glass fiber from optical, environmental and mechanical failure modes. The coating system consists of a soft inner coating, commonly referred to as the inner or primary coating, and a hard outer coating, commonly referred to as the outer or secondary coating. The properties and dimensions of the coating system are designed to minimize microbend induced attenuation losses while maximizing the mechanical protection of the fragile glass surface.
Optical fiber coatings are typically comprised of the reaction product of acrylate functional oligomers and monomers. Because there are a wide variety of oligomers and monomers commercially available, a wide range of coating properties can be achieved. In order to be compatible with the optical fiber draw process, the coatings must polymerize very quickly upon exposure to the UV light in order to avoid the formation of draw-induced defects. Owing to its low modulus and tensile strength, the primary coating is particularly vulnerable to draw-induced defects. The cure speed of the coating can be increased by adding higher levels of photoinitiators; however, as the photoinitiators are generally some of the most expensive components, the result is increased coating cost. Another expensive component of an optical fiber coating formulation is the urethane acrylate oligomer that typically serves as the main component in the coating. Decreasing or eliminating this component would serve to decrease the cost of the coating formulation. Work at Corning has identified low cost thermoplastic elastomers as replacements for the oligomer in an optical fiber primary coating. While the use of thermoplastic elastomers resulted in primary coatings with favorable physical properties, the resulting coatings also displayed slow cure speeds which limited the speed at which the draw could be run (thereby increasing the cost to produce optical fiber). In addition, the high molecular weight of the thermoplastic elastomers limited the level at which they could be added to the coating without increasing viscosity beyond the range preferred for the optical fiber draw process. In regards to secondary coatings, prior work at Corning has focused on the development of secondary coatings in which the urethane acrylate oligomer is substantially or completely replaced by acrylate monomers as a way to decrease coating cost. While this replacement reduces cost, acrylate monomers are still relatively expensive. Therefore a need still exists for low cost optical fiber coatings that exhibit cure rates that are compatible with a high speed draw process.
The primary coating is applied directly to the glass fiber and, when cured, forms a soft, elastic, and compliant material that encapsulates the glass fiber. The primary coating serves as a buffer to cushion and protect the glass fiber core and cladding when the fiber is bent, cabled, or spooled. The secondary coating is applied over the primary coating and functions as a tough, protective outer layer that prevents damage to the glass fiber during processing and use.
Certain characteristics are desirable for the secondary coating. Before curing, the secondary coating composition should have a suitable viscosity and be capable of curing quickly to enable processing of the optical fiber. After curing, the secondary coating should have the following characteristics: sufficient stiffness to protect the encapsulated, glass fiber yet enough flexibility for handling, low water absorption, low tackiness to enable handling of the optical fiber, chemical resistance, and sufficient adhesion to the primary coating.
To achieve the desired characteristics, conventional secondary coating compositions generally contain urethane-based oligomers in large concentration with monomers being introduced into the secondary coating composition as reactive diluents to lower the viscosity. Because conventional oligomeric components are, in general, much more expensive than the monomeric components, the use of oligomers in high concentration has the effect of increasing the cost of producing secondary coating compositions as well as the resulting optical fiber.
Thus, there remains a need for suitable secondary coating compositions which can be prepared at lower cost than conventional secondary coating compositions and yield secondary coatings with a suitable modulus and other physical properties.